This invention relates to coated pipes as used in the oil and gas industry, and particularly to techniques for protecting the coatings of such pipes.
Pipes used in the oil and gas industry are commonly formed of lengths of steel pipe—‘pipe joints’—welded together end-to-end and coated at the resulting field joint as the pipeline is laid. For example, where a pipeline is laid offshore, welding and field joint coating may be performed aboard a pipelaying vessel such as a laybarge that launches the resulting pipe string using S-lay or J-lay methods. It is also common to fabricate a pipe stalk onshore at a spoolbase and to transport the prefabricated pipe offshore for laying in a reel-lay operation, in which welded pipe stalks are stored in a compact spooled form on a pipelaying vessel.
To mitigate corrosion of the pipeline and optionally also to insulate the fluids that the pipeline carries in use, the pipe lengths are pre-coated with protective coatings that, optionally, are also thermally insulating. Many variations are possible in the structure and composition of the coating to obtain the required protective or insulative properties. However, polypropylene (PP) is most commonly used to coat the pipe lengths from which pipelines are made. For example, a three-layer PP (3LPP) coating may be used for corrosion protection and scratch protection and a five-layer PP (5LPP) coating may be used for additional thermal insulation. Additional layers are possible; for example, a seven-layer PP (7LPP) coating may be used where thermal insulation requirements are particularly stringent.
A 3LPP coating typically comprises a fusion-bonded epoxy (FBE) primer applied to the cleaned outer surface of a length of steel pipe. As the primer cures, a second thin layer of PP is applied so as to bond with the primer and then a third, thicker layer of extruded PP is applied over the second layer for mechanical protection. A 5LPP insulation coating typically comprises a succession of PP layers above the FBE primer, those layers being alternately of solid PP and foamed PP with the uppermost layer being of solid PP for mechanical protection of the insulation. A 7LPP insulation coating adds to the 5LPP coating another two layers, namely a sixth layer of foamed PP for additional thermal insulation surrounded by a seventh layer of extruded solid PP for mechanical protection of the insulating sixth layer.
During manufacture of a 5LPP or 7LPP insulation-coated pipe, the coating is laid onto the pipe in a factory by known methods such as wrapping or extrusion. For example, lengths of pipe (typically 12 meters in length) that will form pipe joints may be positioned end-to-end to form an apparently continuous pipe that is then coated on a coating line. At the end of the coating line, the pipe lengths are separated from each other by cutting through the insulation coating.
Later in the same factory, or elsewhere, the edge of the 5LPP or 7LPP insulation coating is cut back from the end of each pipe length and may also be chamfered or bevelled. This leaves a short end portion of pipe with just the FBE anti-corrosion coating at each end of a pipe length to facilitate welding on board a pipelaying vessel for S-lay or J-lay operations or at a spoolbase for reel-lay operations.
The extremity of the pipe is protected temporarily with a plastics end cap that closes the pipe end to protect the interior of the pipe from dust and corrosion. Similarly, once the edge of the 5LPP or 7LPP insulation coating has been cut back, chamfered or bevelled, that edge is protected temporarily by a film tape to protect the edge of the coating—and especially the foamed layers of the coating—from dust and moisture.
The end cap and the film tape are removed as part of the preparations for welding and for field joint coating.
Reference is now made to FIGS. 1 to 3 of the accompanying drawings to illustrate this prior art.
The end caps 16, 18 shown in FIGS. 2 and 3 each comprise a recessed circular central wall 20 surrounded by an annular C-section channel 22 comprising an inner wall 24, an outer wall 26 and an end wall 28. The inner and outer walls 24, 26 are concentrically curved and spaced apart to embrace an edge of the pipe length 10 with a resilient sliding fit that holds the end caps 16, 18 in place by friction. The depth of the inner and outer walls 24, 26 is slightly less than the length of the end portion 30 of the pipe length 10, hence being deeper in the end cap 18 of FIG. 3 where the end portion is longer. This greater depth also helps to secure the end cap 18 against being lost from the end of the pipe length 10 during storage, handling and transportation.
Coated pipe lengths 10 are typically kept in storage or in transit for an extended period before being welded into a pipeline. That period may extend from a few weeks or months to more than a year. During that time, the pipe lengths 10 may be stored outside in harsh, wet or humid environments and may be handled roughly while being transported between various locations. This may damage, or even cause loss of, the temporary protection in the form of the end caps 16, 18 and the film tape 17 applied to the end portion of the pipe length 10 and to the cut edge 14 of the coating 12.
Where end caps 16, 18 are used, they protect the extremity of the pipe length 10 for as long as they remain in place. However, the film tape 17 provides less protection for the cut edge 14 of the coating 12, where the foamed PP layers of the 7LPP structure are not protected by the surrounding solid PP layers as intended. Consequently, during storage, handling and transportation, moisture and dust can penetrate into the foamed PP layers, creating a weak point for future bonding with the field joint coating that is applied between pipe lengths after welding.
Weak bonding introduces a risk that cracks may occur at the interface between the pipe coating and the field joint coating. Any such cracks may allow water to reach the outer surface of the pipe in use, thus corroding the pipe. Water ingress may also reduce the adhesion of the coating to the pipe and may additionally degrade the coating. Clearly, degradation or loss of adhesion of the coating will tend to permit further corrosion of the pipe and may also reduce its thermal insulation. In this respect. a particular problem that may be suffered by a field joint coating of cast-moulded polyurethane (CMPU) is hydrolysis of the PU under heat emanating from within the pipeline in use. The risk of hydrolysis is particularly significant under the high-pressure conditions of deep water.